If “it” is one of the most dangerous words on the Writing section of the SAT, then “do it” is one of the most dangerous phrases. While I’m hesitant to say that any particular word or phrase will be wrong 100% of the time — the College Board can always figure out a new way to test a particular rule — “do it” comes pretty close.
The reason this phrase is so often incorrect is based on the fact that pronouns such as “it” can only refer to nouns or gerunds ( the -ING form a verb), never to any other part of speech such as verbs or adjectives. For example:
Incorrect: People who become vegetarians often do it because they believe strongly in animal rights.
What does the word “it” refer to?
But the gerund “becoming” doesn’t actually show up in the sentence, only the verb “become.” And a pronoun can only refer to a noun or a gerund, not a verb.
The pronoun “it” therefore doesn’t actually refer to anything (in grammatical terms, we can say that it lacks an antecedent), and the sentence cannot be correct. To fix it, we must instead say:
Correct: People who become vegetarians often do so because they believe strongly in animal rights.
On the flipside, anytime you see the phrase “do so” underlined, it is almost certainly correct, and there’s a pretty good chance that the answer to the question will be “No error.”